Seagal is a great figure amongst martial artists. Just forget about his movies (or antics) for a second. He's the first foreigner to ever operate a dojo in Japan, a country where traditions and principles are sky high, if they made an exception for him it probably means he's exceptional. He have many more dojos in different countries around the world and is revered by his students.
Anderson Silva credited him for his front kick ko over Vitor Belfort, and it was pretty much the first time you saw that kind of technique in the UFC back then. Since that ko you see that technique being used a lot more, so like it or not he does have some contribution to the evolution of the sport. And he is also a highly spiritual guy, and some fighters are finding some strenght mentally when they train with him.
So personally I don't understand the hate over this guy, sure he seems like a very weird character but he does have some knowledge that many fighters are looking for